The present invention relates to an unmanned aerial vehicle and a method of launching such a vehicle.
High-altitude, long endurance solar powered aircraft concepts have been proposed for some time. Such vehicles at this altitude provide significant potential benefits. For example, wind strengths and turbulence levels are at a minimum between around 18,000 m and 30,000 m altitude. Additionally, this altitude range is above normal aviation authority certification needs, and large areas of the planet can be observed, with the distance to the horizon being over 500 km. Such unmanned vehicles are therefore suitable for aerial surveys, surveillance and emergency communications in disaster recovery situations.
In 1974, AstroFlight built the first solar powered drone Sunrise I. The promising results of the 10 m span, Sunrise I, led to the Sunrise II, with 4480 solar cells, theoretically capable of attaining a service ceiling of 23,000 m. Sunrise II flew successfully, but broke up in flight at 7,000 m due to a suspected aeroelastic problem.
NASA's High Altitude Solar (HALSOL) project in 1995 saw the flight of the Pathfinder, which reached an altitude of 13,000 m. This was followed by the Pathfinder-Plus which reached 24,500 m, with its 19% efficient silicon solar cells and a payload of 67 kg. Centurion followed with a wing span of 63 m with 62,100 bi-fractal solar cells and a payload of up to 270 kg.
Under NASA's Environmental research Aircraft and Sensor Technology Program (1998-2003) the Centurion was modified to become Helios. The Helios prototype was designed as a proof of concept high-altitude unmanned aerial vehicle that could fly on long endurance environmental science or telecommunications relay missions lasting for weeks or months. Helios made use of 19% efficient silicon based solar cells on the upper wing and lithium batteries. Helios had a constant 2.4 m chord and was assembled in six 12.5 m sections with under wing pods at the juncture of each section. Helios reached an altitude of 29,000 m on solar power. Payload was around 300 kg.
In-flight breakup is thought to be caused by a gust induced aeroelastic wing shape change leading to a control system instability. The resulting pitch oscillation resulted in excessive speeds which caused failure of the wing covering.
In 2005, AC propulsion developed the SoLong aircraft. With the energy storage advances made with Lithium ion batteries (220 WHr/kg), SoLong was able to stay airborne for two half nights, starting with a charged battery at midnight and flying to midnight the next day.
This initial 24 hour flight was followed a few months later with a full 48 hour flight. The present inventor, together with QinetiQ built the Zephyr aircraft (Zephr III) in 2002 in order to film a balloon altitude record attempt at 40,000 m. QinetiQ went on with European funding to develop a ground launch system. In 2007, QinetiQ flew the Zephyr for 83 hours using both 25% efficient solar cells and 350 Whr/kg Lithium Solar batteries. Zephyr reached a record altitude of 29,000 m in 2008. A payload of around 2.5 kg was carried.
The Zephyr holds the official endurance record for an unmanned aerial vehicle for its flight from 9 Jul. to 23 Jul. 2010, lasting 336 hours and 22 minutes (2 weeks/14 days). Record claims have been verified by the Fédération Aéronautique Internationale (FAI) for both duration and altitude. It beat the previous endurance record for unmanned flight by more than a factor of five.
However, these methods of launching an unmanned aerial vehicle at such altitudes involve a ground launching method, whereby the vehicle propels itself from ground level to the relatively calm stratospheric zone above 15-20,000 m.
US 2006/0278757 discloses a method of launching an unmanned aerial vehicle which involves the vehicle being attached to a lighter-than-air carrier from below and does not involve a ground launching approach. Once at the desired altitude the vehicle remains attached to the carrier and begins to fly in an ever-increasingly horizontal attitude, using the carrier as an anchor around which it flies, until it is at a velocity where it is capable of independent flight, whereupon it is separated from the carrier to begin independent and sustained flight.
However, it has been found that such a carrier must carry a significant additional mass, and therefore have a very high lifting force, in order to perform as such an anchor without it itself being moved by the vehicle. It is estimated that the carrier would need a lifting force five to fifteen times that of the weight of the vehicle. If the carrier is a balloon, then such a balloon would have to be very large, e.g 30 m in diameter for a small unmanned aerial vehicle of 75 kg take-off weight.
An improved method of launching an unmanned aerial vehicle into high altitude zones would therefore be highly desirable.